Multimode optical fibers are typically used for short to medium length connections (e.g., local networks). Multimode optical fibers allow the use of relatively inexpensive connectors and light sources, such as vertical cavity surface-emitting lasers (i.e., VCSELs) or light-emitting diodes (i.e., LEDs), which would not be suitable for use with single-mode (i.e., monomode) fibers. Multimode optical fibers, however, present intermodal dispersion, which greatly reduces their bit-rate over lengths greater than a few kilometers, or even a few hundred meters, thus preventing them from being used over long distances.
To reduce intermodal dispersion, the multimode optical fibers used in telecommunications generally have a core with a refractive index that decreases progressively from the center of the fiber to its interface with a cladding. In general, the index profile is described by a relationship known as the “α profile” (or alpha profile), as follows:
      n    ⁡          (      r      )        =      {                                                      n              0                        ·                                          1                -                                  2                  ⁢                  Δ                  ⁢                                                                          ⁢                                                            (                                              r                        a                                            )                                        α                                                                                                            r            ≤            a                                                                          n              0                        ·                                          1                -                                  2                  ⁢                  Δ                                                                                          r            ≥            a                              wherein:                n0 is the refractive index on the optical axis of the fiber;        r is the distance from the optical axis of the fiber;        a is the radius of the core of the fiber;        Δ is a non-dimensional parameter indicative of an index difference between the core and the cladding of the fiber; and        α is a non-dimensional parameter, indicative of the shape of the index profile.        
The parameter Δ is known as the index contrast, and for Δ<<1,n(r≧a)=n0·(1−√{square root over (1−2Δ)})≈Δ·n0. 
Alpha profile fibers and a method of fabricating such optical fibers are disclosed in U.S. Pat. No. 3,989,350, which is hereby incorporated by reference.
The performance of multimode optical fibers has improved to such an extent that they may be used in high bit-rate connections over distances of several hundreds of meters. By way of example, the 10 gigabit (Gb) Ethernet standard (i.e., 10 GbE) provides a profile fibers to be used over distances less than or equal to 300 meters (m).
Typically, manufactured optical fibers (i.e., non-theoretical fibers) present some dispersion from the theoretical α profile. In this regard, the refractive index profile of a fabricated optical fiber may differ slightly from the nominal (i.e., set or theoretical) profile. Unfortunately, transmission properties are sensitive to variations in the refractive index profile. Fabricating graded-index multimode optical fibers thus includes two stages: (i) proper production of the fibers; and (ii) classifying or otherwise evaluating the produced fibers to discard those that do not comply with the specifications.
The real (i.e., actual or non-theoretical) refractive index profile of a multimode optical fiber is rarely measured directly. Typically, it is sufficient to measure the optical fiber's intermodal dispersion at a predefined wavelength λ0. Thus, the previously-mentioned 10 GbE standard requires an effective modal bandwidth (EMB) that is greater than or equal to 2000 megahertz-kilometers (MHz·km) at a wavelength of 850 nanometers (nm). In this regard, the EMB is not, strictly speaking, a bandwidth, but rather the product of a bandwidth multiplied by a propagation distance.
A method of determining EMB is defined in the FOTP-220 standard and its information annexes (e.g., Annex B and Annex D). In brief, the EMB parameter is determined by performing a plurality of individual measurements. Each individual measurement typically includes injecting a spatially localized light pulse into the inlet face of the optical fiber at a predefined radial offset from the axis of the optical fiber (and thus from the center of the face), and determining the time characteristic of the light pulse after propagation through the optical fiber (i.e., from the optical fiber's inlet face to the optical fiber's outlet face). Individual measurements are repeated at different radial offset values. The results of these various individual measurements are combined to determine an effective mode transfer function of the optical fiber, from which the EMB may be determined. To evaluate an optical fiber having a core diameter of 50 microns (i.e., micrometers or μm), the FOTP-220 standard requires 24 individual measurements to be performed.
As used herein, the term “FOTP-220 standard” refers to the document “FOTP-220 differential mode delay measurement of multimode fiber in the time domain” published on Jan. 1, 2003, by the Telecommunications Industry Association (TIA) and identified as information document TIA-455-220-A. The FOTP-220 standard, including its Annexes, is hereby incorporated by reference.
The method of the FOTP-220 standard determines the performance of the optical fiber in terms of bandwidth at a single wavelength only (e.g., 850 nanometers±10 nanometers for the 10 GbE standard).
Using the method of the FOTP-220 standard to determine the optical fiber's performance at a plurality of wavelengths (e.g., over a range of wavelengths), a plurality of independent EMB measurements must be performed (i.e., several tens or indeed several hundreds of individual measurements).
As previously explained, measurements are taken at the time of fabrication to cull optical fibers that do not present the required performance characteristics as a result of fluctuations in their actual index profiles. Making a plurality of individual measurements to determine the optical fiber's performance at multiple wavelengths can greatly increase the cost of producing the optical fiber.
Thus, there remains a need for a low-cost method of determining the performance of a graded-index multimode optical fiber over multiple wavelengths.